This invention relates to electrostatically aided atomization and coating of articles with charged particles. It is disclosed in the context of certain types of coating material dispensers. However, it is believed to be useful in a wide range of coating dispensing applications. As used in this application, terms such as "electrically conductive" and "electrically non-insulative" refer to a broad range of conductivities electrically more conductive than materials described as "electrically non-conductive" and "electrically insulative." Terms such as "electrically semiconductive" refer to a broad range of conductivities between electrically conductive and electrically non-conductive.
In its early years, the field of electrostatically aided coating material atomization and dispensing was dominated by the dispensing of coating materials containing organic solvents. These solvents and the coating materials they carried typically were electrically non-conductive or only very slightly conductive, but the carriers or solvents were also relatively volatile. The particles of these coating materials thus could ordinarily be charged by contact with, or at least passage within relatively short distances of, electrodes maintained at relatively high magnitude potentials with respect to the article(s) to be coated by the atomized coating material particles. However, care needed to be taken not to stimulate high energy electrical discharge across the space between the electrodes and the article(s) being coated. This need dictated considerable attention by operators of such equipment. The volatility of these solvents also raised environmental concerns about the release of so-called voc's (volatile organic compounds).
Efforts have continued to enhance solvent based coating systems, both against the hazards associated with having relatively high magnitude electrical potentials across atmospheres containing voc's, and against the inevitable close proximity of operators to the highly charged electrodes of such equipment. Standards for testing such equipment have been promulgated by a number of testing agencies in various countries. Illustrative of such standards is the Electrostatic Finishing Equipment Approval Standard, Class Number 7260, promulgated by Factory Mutual Research Corporation (the FM standard).
FM standard includes protocols for the testing of both manual equipment (for example, hand held coating atomizing and dispensing guns--the FM standard, section 5) and automatic equipment (for example, atomizers mounted on robot arms--the FM standard, section 6). Among the tests in both cases is a test in which the equipment at operating voltage is probed using a grounded metal sphere having a diameter of one inch (about 2.5 cm). This test takes place in an explosive atmosphere of propane in air. An explosion is a failed test. To achieve FM approval, the equipment must, inter alia, pass this test. Such standards have caused considerable research and improvement in the safety of electrostatic coating systems. Some ways in which the protocols can be addressed are illustrated and described in co-pending U.S. Ser. No. 08/955,039 filed Oct. 21, 1997, titled SAFE CHARGING, and assigned to the same assignee as this application. Other ways also exist. Many of these ways involve the use of electrically non-conductive rotary atomizers. However, many coating applicators prefer electrically conductive, and specifically, metal, rotary atomizers, for example, for their cost, their durability, their ability to transfer charge from the atomizer's connection to a high-magnitude electrostatic potential supply to the atomizer's rotary edge from which the coating material is atomized and dispensed in a cloud toward the article(s) to be coated, and so on. It has been a significant and ongoing challenge of the atomizer industry to address the coating applicators' competing concern for safety and desire for metal rotary atomizers.
This application addresses primarily automatic coating equipment of the type covered by, inter alia, section 6 of the standard, and primarily automatic coating equipment equipped with metal atomizers. One of the tests automatic coating equipment must pass to achieve FM listing is detailed in section 6.3.2.3 of the FM standard. In that test, the automatic protection incorporated into an electrostatic power supply which is to be used to charge the automatic coating equipment is set to its least sensitive position, that is, the position offering the least protection against disruptive electrical discharge. Then, the power supply is cycled off and on while a grounded test probe is maintained at a predetermined spacing from the equipment. This application addresses apparatus to aid metal atomizer-equipped automatic coating equipment in meeting the requirements of section 6.3.2.3 of the FM standard.